The present invention relates to electronic ballasts for realizing high frequency lighting of a fluorescent lamp and a lighting fixture using the same.
Conventional electronic ballasts for fluorescent lamps includes those disclosed in Japanese Patent Publication No. 2005-216553 in which the ballast continuously varies a light output of a fluorescent lamp by operation of a variable resistor for dimming control operation. In such an electronic ballast according to this conventional example, a high frequency output in a inverter circuit is determined depending on a dimming control level determined by a variable resistor for dimming control operation, which is connected to a dimming circuit. A user can obtain a desired light output in a range from a dimming control upper limit to a dimming control lower limit by freely operating the variable resistor for dimming control operation.
Release of electrons from a cathode on one side of a fluorescent lamp is diminished at the end of life stage of the fluorescent lamp and lamp rectification occurs as shown in FIG. 7. At this stage, a voltage drop is increased in the cathode where electrons are hard to be released and power loss is increased in the vicinity of the cathode. As a result, excessive heat is generated in the vicinity of the cathode and a voltage stress applied to a switching element in a inverter circuit is increased, thereby impairing the reliability of the ballast and a lighting fixture with a electronic ballast mounted thereon.
A conventional ballast circuit is shown in FIG. 6 includes means adapted to detect a voltage of a fluorescent lamp FL, wherein an inverter control circuit 3 detects an increase in a voltage applied to the fluorescent lamp FL when a rectification phenomenon occurs at a lamp end of life. This is followed by disabling oscillation of switching elements Q1 and Q2 arranged in inverter circuit 2. However, in this conventional example, it is impossible to determine the difference between an increase in a voltage applied to the fluorescent lamp FL in a dimming control and an increase in a voltage applied to the fluorescent lamp FL at a lamp end of life.
The present invention was achieved by taking the above problems into consideration, having an object to provide a electronic ballast with high reliability by detecting a lamp end of life condition with high accuracy even in the vicinity of a lower limit of a dimming control, without affecting the dimming control level of a fluorescent lamp, and thereby controlling the electronic ballast in a protection mode.